This is the website/blog for Young Pacific ORCA, the first in a pair of ocean bottom seismometer array deployments in the Pacific Ocean. The goal of this ambitious seismic experiment is to image the convecting mantle beneath the Earth’s largest tectonic plate.

Oceanic plates make up 70% of the Earth’s surface and offer important windows into the Earth’s mantle, yet they are largely unexplored due to the lack of seismic data from the ocean basins. The Earth science community has identified several unanswered questions regarding the thermal structure of oceanic plates, the significance of volcanism in the middle of oceanic plates, and how the convecting mantle beneath the plates controls their movements. Undulations in the gravity field and un-explained shallowing of the ocean floors hint that small-scale convection may be occurring beneath the oceanic plates, but this remains unconfirmed.

The Pacific ORCA project contributes to an international effort to strategically place temporary arrays of instruments across the Pacific Ocean basin to record earthquakes worldwide in order to make images of the Earth’s interior. We are collecting unique data from two 30-station arrays in the central and southern Pacific where large gaps in coverage exist today. These arrays, deployed at two distinct plate ages (~30 Ma and ~120 Ma), will address specific critical questions on the dynamics of the oceanic asthenosphere, including its underlying state (temperature, presence of melt, water or other volatiles, and deformation mechanism). The arrays are designed to image the anisotropic velocity signature of small-scale convection, which has been invoked to explain the flattening of the age versus depth curve in old ocean plates, 140-200 km wavelength gravity lineations, and ubiquitous off-axis, non-plume volcanism observed at a variety of scales. Anisotropic surface wave and body wave tomographic models will be supplemented by shear wave splitting and attenuation measurements to obtain a multi-faceted understanding of the asthenosphere and base of the plates. Finally, the order-of-magnitude increases in path coverage for surface and body waves in the south-central Pacific will enable new advances in global tomography.